All of the indicators we track were lower over the past week (March 17-23), compared to the previous 7 days. Average daily sunspot numbers declined from 51.7 to 28.4, and average daily solar flux was 88.8, compared to 98.6 during the prior 7 days. Planetary A index was lower by 3.8 points to 11.9, and average mid-latitude A index (measured in Wallops Island, Virginia) was down from 12 to 8.6. Lower geomagnetic indices are generally considered a good thing for HF propagation.

Looking at the record, we can see that until March 24, there were no new sunspot regions since March 17 when there was one, which followed two on the day before.

Even though our sun is quiet, there is a seasonal variation which produces aurora around the vernal and autumnal equinoxes. (This year, the vernal, or spring, equinox was on Sunday, March 20, and the next autumnal or fall equinox this year will occur on Thursday, September 22. Spaceweather.com posted this article explaining how this works:

From NOAA and the USAF, predicted solar flux is 88, 90, 92, and 88 on March 25-28, 90 on March 29 through April 3, 95 on April 4-11, 90 on April 12-17 and 85 on April 18-20. Flux values are predicted to go back to 95 again after April 30.

Also via NOAA, planetary A index for March 25-29 is predicted at 5, 8, 8, 10, and 8, then 5 on March 30 through April 1, then 30, 20 and 8 on April 2-4, 5 on April 5-7, then 10, 5, 15, 24, 22, and 20 on April 8-13, and 8 on April 14-15. The A index then jumps to 30 on April 29.

F.K. Janda, OK1HH, sends us his geomagnetic forecast, but this week I want to test a different format. You can see the format we’ve been using in past bulletins at http://arrl.org/w1aw-bulletins-archive-propagation. See below for a link to my survey.

Here is the proposed format:

Geomagnetic activity forecast for the period March 25-April 20, 2016

Geomagnetic field will be:

Quiet on March 25-26, April 1, 14, 17-18

Mostly quiet on March 30-31, April 6, 10, 15-16, 19

Quiet to unsettled on March 27, 29, April 7, 20

Quiet to active on March 28, April 5, 8-9, 11, 13

Active to disturbed on April 2-4, 12

Increases in solar wind are expected on

March (30) April 4-6, 9, 11, 14

Remarks:

- Parenthesis means lower probability of activity enhancement.

See examples and vote.

Ray Soifer, W2RS, in Green Valley, Arizona, on March 24 sent this info about working 30 meter gray line propagation yesterday morning:

“Interesting propagation. I heard nothing from VK0EK until half an hour before sunrise. He peaked 579 around sunrise, then faded and was gone 30-40 minutes after. Quite a gray line.”

VK0EK is on Heard Island. The DXpedition started this week. The distance from Ray’s QTH to Heard Island is 10,945 miles — short path. More info about Heard Island, from the Australian government.

Max White, M0VNG, and earlier David Moore sent this link to an ultraviolet image of our Sun.

Martin McCormick, WB5AGZm of Stillwater, Oklahoma sent this piece in a message titled “Is it F2, or E?”

“One of the first things a person notices when listening to HF signals coming from afar is a characteristic sound as if someone was continuously playing with tone controls. It is especially noticeable on AM signals and the wider the receiver passband, the more pronounced is the effect. This is known as selective fading because different frequencies fade in and out at different times.

“The Earth’s ionosphere is a shell or several shells of electrically-charged air molecules and atoms ranging from around 38 miles above us to about 310 miles high. The Sun and cosmic rays cause the molecules and atoms to loose electrons which gives them a positive charge. They can reflect radio signals and give us world-wide radio communication.

“The reflecting layers are the E layer at about 56-93 miles plus the F layer complex which consists of several shells or layers that come, go and move around based on Solar and Geo magnetic activity.

“The F2 layer is mostly responsible for short wave communications and the selective fading is due to more than one layer reflecting the same signal back to Earth. If the layers happen to be half of a wavelength apart, one receives two versions of the same signal in which one version is delayed enough to cause the carrier of an AM or FM signal to be cancelled out. Mother Nature just turned a normal AM signal in to a double-sideband suppressed carrier transmission.

“FM signals are also altered. It is possible to receive FM on an AM receiver by tuning slightly off from center. It is called slope detection and has existed for as long as there has been FM. If you tune right on to the frequency, the audio is faint and muffled. F2 propagation, however, can cause a selective fade just below or above the carrier frequency such that if you are on the center frequency, the amplitude of the signal matches the deviation and you can hear fairly good audio as if the signal was both AM and FM. Nature automatically did that, also but the effect may not last more than a fraction of a second.

“When Sporadic E is involved, signals still fade up and down but the E layer acts like a single thin film and there is no selective fading. Except for fades, signals sound local.”

For more information concerning radio propagation, see the ARRL Technical Information Service at http://arrl.org/propagation-of-rf-signals. For an explanation of the numbers used in this bulletin, see http://arrl.org/the-sun-the-earth-the-ionosphere. An archive of past propagation bulletins is at http://arrl.org/w1aw-bulletins-archive-propagation. More good information and tutorials on propagation are at http://k9la.us/.

My own archives of the NOAA/USAF daily 45 day forecast for solar flux and planetary A index are in downloadable spreadsheet format at http://bit.ly/1VOqf9B and http://bit.ly/1DcpaC5.

Click on “Download this file” to download the archive, and ignore the security warning about file format. Pop-up blockers may suppress the download.

Monthly propagation charts between four USA regions and twelve overseas locations are at http://arrl.org/propagation.

Instructions for starting or ending email distribution of ARRL bulletins are at http://arrl.org/bulletins.

Sunspot numbers for March 17 through 23 were 66, 29, 26, 26, 25, 13, and 14, with a mean of 28.4. The 10.7 cm flux was 91.6, 90.3, 89.4, 87.6, 88.9, 87.3, and 86.8, with a mean of 88.8. Estimated planetary A indices were 21, 8, 18, 10, 8, 8, and 10, with a mean of 11.9. Estimated mid-latitude A indices were 16, 6, 12, 6, 7, 6, and 7, with a mean of 8.6.